1) Field of the Invention
The present invention relates to non-destructive inspection and, more particularly, to non-destructive inspection of a structure for defects using an impact echo method.
2) Description of Related Art
Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the integrity and fitness of the structure. In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies and structures with contoured surfaces. These composite structures, and a shift toward lightweight composite and bonded materials such as using graphite materials, dictate that devices and processes are available to ensure structural integrity, production quality, and life-cycle support for safe and reliable use. As such, it is frequently desirable to inspect structures to identify any defects, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. For example, typical defects in composite sandwich structures, generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, include disbonds which occur at the interfaces between the core and the skin or between the core and a buried septum.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse-echo, or mechanical impedance sensors are typically used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure is commonly performed using ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. Data acquired by sensors is typically processed and then presented to a user via a display as a graph of amplitude of the received signal. To increase the rate at which the inspection of a structure is conducted, a scanning system may include arrays of inspection sensors, i.e., arrays of transmitters and/or detectors.
Non-destructive inspection may be performed manually by technicians who typically move an appropriate sensor over the structure. Manual scanning requires a trained technician to move the sensor over all portions of the structure needing inspection. Manual scanning typically involves the technician repeatedly moving a sensor side-to-side in one direction while simultaneously indexing the sensor in another direction. In addition, because sensors typically do not associate location information with the acquired data, the same technician who is manually scanning the structure must also watch the sensor display while scanning the structure to determine where the defects, if any, are located in the structure. The quality of the inspection, therefore, depends in large part upon the technician's performance, not only regarding the motion of the sensor, but also the attentiveness of the technician in interpreting the displayed data. Thus, manual scanning of structures is time-consuming, labor-intensive, and prone to human error.
Semi-automated inspection systems have also been developed. For example, the Mobile Automated Scanner (MAUS®) system is a mobile scanning system that generally employs a fixed frame and one or more automated scanning heads typically adapted for ultrasonic inspection. A MAUS system may be used with pulse-echo, shear-wave, and through-transmission sensors. The fixed frame may be attached to a surface of a structure to be inspected by vacuum suction cups, magnets, or like affixation methods. Smaller MAUS systems may be portable units manually moved over the surface of a structure by a technician.
Automated inspection systems have also been developed. For example, the Automated Ultrasonic Scanning System (AUSS®) system is a complex mechanical scanning system that may employ through-transmission ultrasonic inspection. An AUSS system can also perform pulse-echo inspections, and simultaneous dual frequency inspections. The AUSS system has robotically controlled probe arms that may be positioned, for example, for TTU inspection proximate the opposed surfaces of the structure undergoing inspection with one probe arm moving an ultrasonic transmitter along one surface of the structure, and the other probe arm correspondingly moving an ultrasonic receiver along the opposed surface of the structure. To maintain the ultrasonic transmitter and receiver in proper alignment and spacing with one another and with the structure undergoing inspection, a conventional automated inspection system may have a complex positioning system that provides motion control in numerous axes, such as the AUSS-X system which has motion control in ten axes. Automated inspection systems and like robotics, however, can be prohibitively expensive. Further, orienting and spacing sensors with respect to the structure, and with respect to one another for TTU inspection, may be especially difficult in conjunction with structures with non-planar shapes, such as the inspection of curved structures and hat stringers. Also, conventional automated scanning systems, such as the AUSS-X system, may require access to both sides of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for very large or small structures. Furthermore, scanning systems inspect limited areas up to a few meters square.
Other inspection techniques have been utilized to inspect non-composite and non-metallic structures. For example, a technique used to inspect concrete and masonry materials is the impact-echo method. The impact-echo method was developed in the 1980's where a small sphere was used to generate short duration mechanical impact. It was discovered that by varying the diameter of the sphere and the impact force of the sphere, stress waves could be generated through concrete structural elements to discover internal flaws and defects. The time-of-flight diffraction technique includes impacting the concrete to generate longitudinal (P), shear (S), and surface (R) waves. The location of a defect may be determined by monitoring the speed of the stress waves. In particular the P and S waves are reflected by internal defects or external boundaries. When the reflected waves, or echoes, return to the surface, they produce displacements that are measured by a receiving transducer. The impact-echo technique typically requires an impactor to generate stress waves, a receiving transducer unit, an A/D-converter, and a computer installed with waveform analysis software to process the collected signals. A further discussion of the impact-echo method is disclosed in “The Impact-Echo Method,” by Mary J. Sansalone and William B. Street, NDTnet, The Online Journal of Nondestructive Testing & Ultrasonics, Vol. 3, No. 2 (February 1998), which is incorporated herein by reference.
Despite utilizing impact-echo techniques to conduct non-destructive inspection of concrete and masonry structures, improvements are desired to inspect for various defects within a structure. In particular, an inspection system that is versatile and capable of being used for non-destructive inspection beyond concrete and masonry structures, such as for composites and metallic structures, is desired. Because composites and metallic structures used, for example, in the airline industry are typically much thinner than concrete and masonry structures tested by the impact-echo method, the stress waves mainly travel along the surface of the structure. Therefore, the collection and analysis of the stress waves through the metallic or composite structure is unlike those of stress waves traveling through concrete or masonry structures.
It would therefore be advantageous to provide a non-destructive inspection system that is capable of employing the impact-echo method for materials other than concrete or masonry. In addition, it would be advantageous to provide an inspection system that is portable and capable of inspecting structures effectively and efficiently. Furthermore, it would be advantageous to provide a non-destructive inspection system that is economical to manufacture and use.